1. Field of the Invention
The present invention relates to a saddle anchorage in which the direction of the prestress is not perpendicular to the compression surface of the concrete structure, and a mounting method thereof.
2. Description of the Prior Art
Ever since the prestressed concrete engineering was put into practice in 1940, post-tensioning tendons have been utilized in the interior of the walls of many large prestressed concrete structures, such as bridges, oil tanks, cement silos, and grain silos, to support the dead loads thereof and other loads. After the prestressed concrete structure is built, the action forces, e.g. external loads, live loads, and temperature variation actions, are mainly supported by such post-tensioned tendons.
In general, the stressing end anchorage of tendons is mounted onto the end surface of the wall of the structure. However, under many circumstances, the stressing end anchorage has to be mounted onto a side wall (a web) or a bottom wall (a slab) of the structure. In these cases, it is necessary to further construct a plurality of concrete buttresses projecting from the web or the slab of the structure to carry these stressing end anchorages. For example, when tendons are installed in the interior of the web of a cylindrical structure, such as an oil tank, a cement silo, or a grain silo, to provide a circumferential prestress, it is necessary to construct a plurality of triangular conical or rectangular concrete buttresses onto the web of the structure to carry the stressing end anchorages. Also, as shown in FIG. 1, when the free cantilever method is used to construct a bridge, it is necessary to install post-tensioning tendons in the top and bottom parts of the box girders to support different loads. Here, tendons H (post-tensioning tendons for negative moments) are installed in the webs of the box girders to undertake the negative moments created by the dead loads of the box girders and other loads when the working wagon moves outwards from the pier during construction, while tendons L (post-tensioning tendons for positive moments) are installed in the webs or the slabs at the outside end of the end span and at the middle span to undertake the positive moments created by part of the dead loads of the box girders and other loads when the box girders are closed to complete the bridge construction. Since tendons H are anchored on the outwardly facing end surfaces of the webs of new segments, the mounting of the stressing end anchorages is straightforward and relatively easy. However, since tendons L are anchored on the side surfaces of the webs of the box girders, it is necessary to construct a plurality of triangular conical concrete buttresses onto the side surfaces of the webs to carry these stressing end anchorages.
The inventors have worked in the field of the prestressing concrete engineering for more than ten years, and have been involved in the circumferential prestressing of cylindrical structures and the construction of bridges using the free cantilever method. The inventors have encountered the undesirable time and labor consuming in further constructing the triangular conical buttress to carry the stressing end anchorage on the side surface of the web. As shown in FIG. 2, in the conventional method of constructing the triangular conical buttress, a triangular conical reinforcing cage T which protrudes gradually along the longitudinal direction of the web W is set up by tying steel bars to the ones of the web W. After a sheath P for strands, an anchor body EA connected to the end of the sheath P, and a spiral reinforcement EP (to resist the tensile stress) wrapped around the outer circumference of the anchor body EA are buried into the interior of the triangular conical reinforcing cage T, a triangular conical formwork (not shown) is constructed around the triangular conical reinforcing cage T. Then, concrete is cast into the formwork. After the concrete is cured to form the desired triangular conical buttress B, steel tendons (not shown) are inserted into the sheath P and post-tensioned, and the post-tensioned ends are fixed afterwards to the anchor body EA.
From previous experiences, it is known that, when a bridge is constructed by the free cantilever method, an additional time of 1 to 2 working days is needed to construct a triangular conical buttress B on the side surface of the web of the box girder. This is because that, in order to construct the triangular conical reinforcing cage T which protrudes gradually along the longitudinal direction of the web W of the box girder, it is necessary to tie a plurality of steel bars Tt bent into a shape and a plurality of straight steel bars Tx different in length with one another. However, since the steel bars Tt and Tx are to be tied at different locations on the reinforcing cage T, their lengths and bending positions are different from one another. Hence, these steel bars Tt and Tx can not be mass-produced in standardized dimensions. Each time a reinforcing cage is needed, steel bars are cut and bent into segments of desired lengths and shapes, which is a very troublesome work, and the tying of the steel bars to construct the triangular conical reinforcing cage and to mount the cage onto the side surface of the web requires skilled workers. Furthermore, to set up the triangular conical formwork around the triangular conical reinforcing cage is time consuming and usually wastes a considerable amount of forms. Finally, to form a triangular conical buttress B, an extra 1 to 2 cubic meters of concrete is needed, making it uneconomical.
In view of the various problems presented in the prior art, the present invention provides a saddle anchorage which is manufactured in the factory before taken to the construction site and can be mounted directly onto a side surface of a web or a slab of a prestressed concrete structure, and a mounting method of the saddle anchorage, thereby eliminating the on-site tying of a reinforcing cage, reducing the amount of wasted concrete and forms, and solving other problems of prior art.